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From 1986 to 1988, 14 strands of Norway spruces have been studied in various "Ecological territories" in Belgium. In each strand, 10 trees were choosen randomly and baited with pheromone bags. After the development of the beetle galleries, bark samples were taken in view of quantifying the numbers of attacks and their development.
The numbers of attacks varied strongly in function of the height on the tree, the proximity of the pheromone bag appeared to be a decisive determining factor. On the contrary, the orientation had a low influence, the southern face of the tree was slightly preferred.
The variability between the numbers of attacks, and between the galeries development, gave raise to the hypothesis of the existence of a variability in the resistance of the trees and in the aggressiveness of the beetles, and that the tree possesses several defense barriers which are used successively according the progress of the insects attack.
The numbers of attacks in the tree samples couldn't be fitted to a Poisson distribution but formed an aggregative one, where a number of trees possessed a high resistance. These facts suggest the possibility to select strains of trees resistant to Ips typographus.